Packaging a sealed cavity in an electronic device

ABSTRACT

An electronic device includes a package substrate, a circuit assembly, and a housing. The circuit assembly is mounted on the package substrate. The circuit assembly includes a first sealed cavity formed in a device substrate. The housing is mounted on the package substrate to form a second sealed cavity about the circuit assembly.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.15/696,245 filed Sep. 6, 2017, the entirety of which is incorporatedherein by reference.

BACKGROUND

Various applications require that the integrity of a sealed chamber bemaintained to insure proper equipment operation. For example, for ahousing intended to maintain a low internal pressure, a leak in thehousing may allow ingress of gas that dilutes or contaminates thecontents of the housing to the extent that housing contents are nolonger useable for the intended purpose. In one particular application,the waveguide of a chip-scale atomic clock contains a selected vapor andrequires that a predetermined rate of leakage be maintained to insurethe pressure of the vapor and proper operation of the clock.

SUMMARY

Techniques for controlling and monitoring leakage into a sealed cavityof a semiconductor device are described herein. In one embodiment, anelectronic device includes a package substrate, a circuit assembly, anda housing. The circuit assembly is mounted on the package substrate. Thecircuit assembly includes a first sealed cavity formed in a devicesubstrate. The housing is mounted on the package substrate to form asecond sealed cavity about the circuit assembly.

In another embodiment, a clock generator includes a first hermeticallysealed cavity, clock generation circuitry, and a housing. The firsthermetically sealed cavity is formed in a first substrate. The firsthermetically sealed cavity contains dipolar molecules. The clockgeneration circuitry is configured to drive a signal into the firsthermetically sealed cavity, and to generate an output clock signal at afrequency of quantum rotational state transition of the dipolarmolecules. The housing encloses the first hermetically sealed cavity andthe clock generation circuitry. The housing forms a second hermiticallysealed cavity.

In a further embodiment, an electronic device includes a packagesubstrate, a circuit assembly, a first pressure sensor, a secondpressure sensor, and a housing. The circuit assembly is mounted on thepackage substrate. The circuit assembly includes a first sealed cavity.The first sealed cavity includes a channel formed in a device substrateand a sealing plate bonded to the device substrate. The first pressuresensor is coupled to the sealing plate and is configured to measurepressure within the first sealed cavity as a function of displacement ofthe sealing plate. The housing is mounted on the package substrate toform a second sealed cavity about the circuit assembly. The secondpressure sensor is coupled to the housing and configured to measurepressure within the second sealed cavity as a function of displacementof the housing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a diagram of an electronic device packaged to provide verylow leakage to a chip scale cavity in accordance with variousembodiments.

FIG. 2 illustrates leakage conditions in the electronic device of FIG. 1in accordance with various embodiments.

FIG. 3 shows a relationship of the leakage rate of an outer cavity tothe leakage rate of an inner cavity, where the inner cavity is disposedwithin the outer cavity of an electronic device in accordance withvarious embodiments.

FIG. 4 shows the relationship of the volume of an outer cavity to theleakage rate of an inner cavity, where the inner cavity is disposedwithin the outer cavity of an electronic device in accordance withvarious embodiments.

FIG. 5 shows pressure over time for a first cavity that is not enclosedwithin a second outer cavity.

FIG. 6 shows pressure over time for an inner cavity and an outer cavity,where the inner cavity is disposed within the outer cavity of anelectronic device in accordance with various embodiments.

FIG. 7 shows a diagram of an electronic device packaged to provide verylow leakage to a chip scale cavity that includes sensors for monitoringthe pressure in cavities of the device in accordance with variousembodiments.

FIG. 8 shows pressure related displacement of a sealing plate thathermetically seals a cavity formed in a semiconductor substrate inaccordance with various embodiments.

FIG. 9 shows resonant signature tracking employed to determine thepressure within an inner or outer cavity of an electronic device inaccordance with various embodiments.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

In this description, the term “couple” or “couples” means either anindirect or direct wired or wireless connection. Thus, if a first devicecouples to a second device, that connection may be through a directconnection or through an indirect connection via other devices andconnections. Also, in this description, the recitation “based on” means“based at least in part on.” Therefore, if X is based on Y, then X maybe a function of Y and any number of other factors.

Maintaining reliable operation of a millimeter-wave chip scale atomicclock (mmWCSAC) over a reasonable lifetime (e.g., 3-10 years) requiresthat portions of the clock operate in a stable, hermetically sealedenvironment. For example, reliable operation over time may require awafer scale hermetic seal with a very low leakage rate (e.g., 1E-14atm-cc/s). Unfortunately, building and validating wafer scale hermeticcavities with such low leakage rates is beyond the capabilities ofconventional packaging techniques. Even if such systems can befabricated in the future, testing hermeticity to such levels remains atthe very limits of conventional test facilities. If pressure or humiditywithin the mmWCSAC drifts too far from the manufacturing specifications,the accuracy of the mmWCSAC may be affected. Therefore the hermeticityand packaging are important to enabling mmWCSAC technology.

The electronic devices described herein provide a very low leakage rateto a cavity formed in a semiconductor material. The hermetic sealassociated with such a cavity is incapable of supporting the requiredleakage rate. Rather, the electronic devices include a housing thatsurrounds the cavity and circuitry associated with cavity. The housingprovides a secondary hermetic seal that operates in conjunction with thehermetic seal associated with the cavity to provide the very low leakagerate. Embodiments of the electronic devices described herein alsoinclude sensors that measure the pressure in the cavity and sensors thatmeasure the pressure in the housing. In various embodiments of theelectronic devices, the sensors are strain gauges or acoustictransducers.

FIG. 1 shows a diagram of an electronic device 100 packaged to providevery low leakage to a chip scale cavity in accordance with variousembodiments. In some embodiments, the electronic device 100 is a clockgenerator, such as mmWCSAC. The electronic device 100 includes a packagesubstrate 102, a circuit assembly 103, and a housing 110. The circuitassembly 103 is mounted to the package substrate 102. The packagesubstrate 102 is formed of ceramic or other suitable material. Thecircuit assembly 103 includes a device substrate 104, a sealing plate112, and control circuitry 108. The device substrate 104 is bonded tothe package substrate 102. The device substrate 104 includes a cavity106. The cavity 106 is formed by etching a channel into the devicesubstrate 104. In some embodiments, for example embodiments of ammWCSAC, the surfaces of the cavity 106 are plated with metal to form awaveguide and the cavity contains dipolar molecules. For example, someembodiments of the cavity 106 contain water molecules in vapor form. Theinternal pressure of the cavity 106 is set to an optimum value atmanufacture by controlling the number of the dipolar molecules presentin the cavity 106. In embodiments of the electronic device 100 thatimplement a mmWCSAC, the frequency of quantum rotational statetransition of the dipolar molecules serves as frequency reference for aclock signal generated by the device 100. If the pressure within thecavity 106 varies from the optimum value set at manufacture by anexcessive amount, then the accuracy of the clock signal may be affected.

The sealing plate 112 is bonded to the device substrate 104 tohermetically seal the cavity 106. The sealing plate 112 is a dielectric(e.g., glass) membrane in some embodiments. The control circuitry 108 isbonded to metal plated to the top of the sealing plate 106. Inembodiments of the electronic device 100 that implement a mmWCSAC, thecontrol circuitry 108 includes clock generation circuitry to drivesignal into the cavity 106, to receive signal from the cavity 106, andto process the signal received from the cavity 106 to generate a clocksignal that is locked to the frequency of quantum rotational statetransition of the dipolar molecules contained in the cavity 106.

To provide a device operational life of at least three years,embodiments of the electronic device 100 ensure a very low rate ofleakage into the cavity 106. The principal path for leakage thatdegrades the performance of the electronic device 100 is from theambient environment into the cavity 106. If it is assumed that theperformance of the electronic device 100 is acceptable with a doublingof pressure within the cavity 106, then an acceptable leakage rate forthe cavity 106 can be determined. The leakage rate for the cavity 106alone can be determined as:

$\begin{matrix}{{\Delta\; P} = {( {P_{ext} - P_{int}} )( {1 - e^{{- L}\;{t/V}}} )}} & (1)\end{matrix}$where:

ΔP is change in pressure in the cavity over time;

P_(ext) is pressure external to the cavity;

P_(int) is internal pressure of the cavity;

L is rate of leakage into the cavity;

V is volume of the cavity; and

t is time.

The cavity 106 alone must have an extremely low leakage rate to maintainintra-cavity pressure within an acceptable range. Embodiments of theelectronic device 100 allow the leakage rate of the cavity 106 to berelaxed by disposing the housing 110 about the circuit assembly 103. Insome embodiments of the electronic device 100, the housing 110 ismounted to the package substrate 102 to form a hermetically sealedcavity 114 about the device substrate 102 and control circuitry 108.Thus, in the electronic device 100, the cavity 106 formed in the devicesubstrate 104 is enclosed within an outer cavity 114 formed by thehousing 110. In some embodiments, the housing 110 is metal, glass, oranother material, or combination of materials. For the nested cavitiesof the electronic device 100, the leakage rates of the two cavities 106and 114 are coupled as:

$\begin{matrix}{{\Delta\; P_{1}} = {( {P_{ext} - P_{1}} )( {1 - e^{{- L_{1}}\;{t/V_{1}}}} )}} & (2) \\{{\Delta\; P_{2}} = {( {P_{1} - P_{2}} )( {1 - e^{{- L_{2}}\;{t/V_{2}}}} )}} & (3)\end{matrix}$where:

ΔP₁ is change in pressure in the outer cavity 114;

P_(ext) is pressure external to the outer cavity 114;

P₁ is internal pressure of the outer cavity 114;

L₁ is rate of leakage into the outer cavity 114;

V₁ is volume of the outer cavity 114;

ΔP₂ is change in pressure in the inner cavity 106;

P₂ is internal pressure of the inner cavity 106;

L₂ is rate of leakage into the inner cavity 106;

V₂ is volume of the inner cavity 106; and

t is time.

FIG. 2 illustrates leakage conditions in the electronic device 100. Thepressure of the atmosphere (P_(atm)) external to the outer cavity 114induces leakage L₁ into the outer cavity 114. The outer cavity 114 hasvolume V₁ and internal pressure P₁. The inner cavity 106 has volume V₂and internal pressure P₂. The difference in pressures of the internaland external cavities induces leakage L₂ from the outer cavity 114 intothe inner cavity 106. Given values for the volumes of the cavities 106and 114 and average change in pressure in the outer cavity 114 (ΔP₁),the relationship between leakage rates and volumes of the cavities 106and 114 can be determined.

FIG. 3 shows the relationship of the leakage rate of the inner cavity106 to the leakage rate of the outer cavity 114. FIG. 3 shows that toachieve a given overall leakage rate into the inner cavity 106, theindividual leakage rate of the inner cavity 106 may be increased as theleakage rate of the outer cavity 114 is decreased. Similarly, to achievethe given overall leakage rate, the individual leakage rate of the innercavity 106 must be decreased as the leakage rate of the outer cavity 114is increased.

FIG. 4 shows the relationship of the leakage rate of the inner cavity106 to the volume of an outer cavity 114. FIG. 4 shows that to achieve agiven overall leakage rate into the inner cavity 106, the individualleakage rate of the inner cavity 106 may be increased as the volume ofthe outer cavity 114 is increased. Similarly, to achieve the givenoverall leakage rate, the individual leakage rate of the inner cavity106 must be decreased as the volume of the outer cavity 114 isdecreased.

To maintain pressure suitable for operation of a mmWCSAC in the cavity106, without the outer cavity 114 formed by the housing 110, a leakagerate on the order of 1E-14 atm-cc/s is required. FIG. 5 shows pressureover time for the inner cavity 106 without the outer cavity 114. FIG. 5shows that given an initial pressure of 10⁻⁴ atm in the cavity 106 and aleakage rate of 1E-14 atm-cc/s into the cavity 106, without the outercavity 114, the pressure within the cavity 106 doubles in less than 3.5years. As previously noted, a leakage rate of 1E-14 atm-cc/s isdifficult or impossible to achieve with conventional manufacturingtechniques.

FIG. 6 shows pressure over time in the inner cavity 106 with the outercavity 114 as in the electronic device 100. In FIG. 6, the leakage rateinto the outer cavity 114 is 1E-10 atm-cc/s and the leakage rate intothe inner cavity 106 is 1E-11 atm-cc/s. The initial pressure in thecavities 106 and 114 is 10⁻⁴ atm. FIG. 6 shows that the in theelectronic device 106 the pressure within the cavity 106 doubles in justover four years. Accordingly, embodiments of the electronic device 100allow the effective leakage rate of the cavity 106 to be greatly reduced(e.g., by three orders of magnitude or more (e.g., 1000, 5000, etc.))relative to a device that lacks the outer cavity 114. Thus, theelectronic device 100 provides a reasonable operating life whilerelaxing the requirements of individual cavity hermeticity by a factorof 1000 or more and allowing the device's hermetic seals to bemanufactured using conventional processes.

Some embodiments of the electronic device 100 also include features formonitoring the pressure within the inner cavity 106 and the outer cavity114. Monitoring cavity pressure allows the operational state of theelectronic device 100 to be verified under operating conditions. If thepressure in the cavity 106 exceeds a predetermined level, then someembodiments generate a notification signal to alert higher level systemsof the condition of the device 100. FIG. 7 shows a diagram of anelectronic device 700 packaged to provide very low leakage to a chipscale cavity that includes sensors for monitoring the pressure in thecavities of the device in accordance with various embodiments. Theelectronic device 700 is an embodiment of the electronic device 100.

The electronic device 700 includes package substrate 102, a circuitassembly 103, a housing 110, and pressure sensors 702 and 704. Thecircuit assembly 103 is mounted to the package substrate 102. Thecircuit assembly 103 includes a device substrate 104, a sealing plate112, and control circuitry 108. The device substrate 104 is bonded tothe package substrate 102. The device substrate 104 includes a cavity106. The cavity 106 is etched into the device substrate 104. In someembodiments, the surfaces of the cavity 106 are plated with metal toform a waveguide and the cavity contains dipolar molecules. For example,some embodiments of the cavity 106 contain water molecules in vaporform. The internal pressure of the cavity 106 is set to an optimum valueat manufacture by controlling the number of the dipolar moleculespresent in the cavity 106. In embodiments of the electronic device 100that implement a mmWCSAC, the quantum rotation frequency of the dipolarmolecules serves as frequency reference for a clock signal generated bythe device 100. If the pressure within the cavity 106 varies from theoptimum value set at manufacture by an excessive amount, then theaccuracy of the clock signal may be affected.

The sealing plate 112 is bonded to the device substrate 104 tohermetically seal the cavity 106. The sealing plate 112 is a dielectric(e.g., glass) membrane in some embodiments. The control circuitry 108 isbonded to metal plated to the top of the sealing plate 106. Inembodiments of the electronic device 700 that implement a mmWCSAC, thecontrol circuitry 108 includes clock generation circuitry to drivesignal into the cavity 106, to receive signal from the cavity 106, andto process the signal received from the cavity 106 to generate a clocksignal that is locked to the quantum rotation frequency of the dipolarmolecules contained in the cavity 106.

Embodiments of the electronic device 700 allow the leakage rate of thecavity 106 to be relaxed by disposing the housing 110 about the devicesubstrate 104. In some embodiments of the electronic device 100, thehousing 110 is mounted to the package substrate 102 to form ahermetically sealed cavity 114 about the device substrate 102 andcontrol circuitry 108. Thus, in the electronic device 700, the cavity106 formed in the device substrate 104 is enclosed within an outercavity 114 formed by the housing 110. In some embodiments, the housing110 is metal, glass, or another material or combination of materials.

The pressure sensors 702 and 704 are disposed to measure the pressure inthe inner cavity 106 and the outer cavity 114 respectively. For example,the pressure sensor 702 is coupled to the sealing plate 112 to measurepressure within the inner cavity 106, and the pressure sensor 704 iscoupled to the housing 110 to measure pressure within the outer cavity114. The pressure sensor 702 is coupled to the sealing plate 112 at alocation in which the inner cavity 106 is adjacent to the sealing plate112. Similarly, the pressure sensor 704 is coupled to the housing 110 ata location of the housing 110 most likely to be affected by a change inpressure of the outer cavity 114.

In some embodiments of the electronic device 700, the pressure sensors702 and 704 are strain gauges that measure the deflection ordisplacement of the sealing plate 112 and the housing 110 respectively.Displacement of the sealing plate 112 is representative of the pressurewithin the inner cavity 106. For example, as the forces exerted on thesealing plate 112 by the pressures in the inner cavity 106 and the outercavity 114 change, the displacement of the sealing plate 112 alsochanges. FIG. 8 shows an example of displacement of the sealing plate112 that is measureable by the pressure sensor 702.

Some embodiments of the control circuitry 108 include circuitry toreceive signals from the pressure sensors 702 and 704, and to determinevalues of pressure in the inner cavity 106 and the outer cavity 114based on the signals. For example, circuits in the control circuitry 108digitize the signals received from the pressure sensors 702 and 704, andaccess a table that relates displacement values to cavity pressurevalues, or evaluate a function that produces cavity pressure valuesbased on the displacement values. In such embodiments, the controlcircuitry 108 includes an analog-to-digital converter to digitize thesignals generated by the pressure sensors 702 and 704, a microcontrollerand associated programming to generate pressure values based on thedigitized signals, or other circuitry suitable for generating pressurevalues based on displacement signals.

In some embodiments of the electronic device 700, the pressure sensors702 and 704 are acoustic transducers that detect vibration of thesealing plate 112 and the housing 110 respectively. The resonantfrequencies of the sealing plate 106 and the housing 110 arerepresentative of the pressures within the inner cavity 106 and theouter cavity 114. For example, as the pressure within the inner cavity106 increases the resonant frequency of the sealing plate 112 alsoincreases. Similarly, as the pressure within the outer cavity 114increases, the resonant frequency of the housing 110 increases. FIG. 9shows an example of tracking of the resonant frequency of the sealingplate 112.

Some embodiments of the control circuitry 108 include circuits toreceive signals from the pressure sensors 702 and 704, and to determinevalues of pressure in the inner cavity 106 and the outer cavity 114based on the signals. For example, circuits in the control circuitry 108digitize the signals received from the pressure sensors 702 and 704, andanalyze the frequency content of the signals to determine the mechanicalharmonic signature of the sealing plate 112 and/or the housing 110. Thecontrol circuitry 108 accesses a table that relates harmonic signatureto cavity pressure values, or evaluates a function that produces cavitypressure values based on harmonic signature. In such embodiments, thecontrol circuitry 108 includes an analog-to-digital converter todigitize the signals generated by the pressure sensors 702 and 704, amicrocontroller and associated programming, or other suitable circuitry,to analyze the frequency content of the digitized signals, determine aharmonic signature of the sealing plate 112 and/or the housing 110, anddetermine a value of pressure based on the harmonic signature.

Modifications are possible in the described embodiments, and otherembodiments are possible, within the scope of the claims.

What is claimed is:
 1. An electronic device, comprising: a packagesubstrate; a circuit assembly mounted on the package substrate, thecircuit assembly including a first cavity formed in a device substrate;a sealing plate attached to the device substrate, enclosing the firstcavity and configured to maintain a first pressure within the firstcavity; and a housing attached to the package substrate forming a secondcavity about the circuit assembly, and configured to maintain a secondpressure within the second cavity.
 2. The electronic device of claim 1,wherein the first cavity includes a channel formed in the devicesubstrate.
 3. The electronic device of claim 1, further comprising apressure sensor coupled to the sealing plate and configured to measurepressure within the first cavity as a function of displacement of thesealing plate.
 4. The electronic device of claim 1, wherein the sealingplate includes a dielectric membrane.
 5. The electronic device of claim1, including: an acoustic sensor coupled to the sealing plate andconfigured to measure vibration of the sealing plate; and controlcircuitry coupled to the acoustic sensor, the control circuitryconfigured to determine pressure within the first cavity as a functionof the mechanical harmonic signature of the sealing plate.
 6. Theelectronic device of claim 1, including a pressure sensor coupled to thehousing and configured to measure pressure within the second cavity as afunction of displacement of the housing.
 7. The electronic device ofclaim 1, including: an acoustic sensor coupled to the housing andconfigured to measure vibration of the housing; and control circuitrycoupled to the acoustic sensor, the control circuitry configured todetermine pressure within the second cavity as a function of themechanical harmonic signature of the housing.
 8. The electronic deviceof claim 1, wherein the first cavity is a waveguide of a millimeter-wavechip scale atomic clock.